This invention relates to improvements in methods and circuits for controlling a velocity of a read/write head of a disk drive, and more particularly, to obtaining precise control of the movement of a voice coil motor by measuring a back electromotive force (BEMF) voltage generated in the voice coil.
FIG. 1 shows a mass storage device system 1, such as a computer hard disk, compact disk read only memories (CD-ROMs), digital video disks (DVDs), etc., recording and/or reading head 3 is typically positioned at selective locations radially outward from the center of a rotating data medium 2 when writing or reading digital data to or from the data medium. The head 3 is positioned by selectively applying controlled current to a voice coil motor 6 to move an arm (or head mechanism) that carries the head.
Under certain circumstances, such as when disk rotation is stopped, the head 3 is positioned to a xe2x80x9cparkedxe2x80x9d position 4 by moving the head mechanism up a ramp 5 assembly so that the head is maintained at a position away from the disk. By parking the head 3 in this external safe zone, the reliability of the disk drive against shock and vibrations during its non-operative time is increased. This is particularly important in xe2x80x9claptopxe2x80x9d or portable computers that may be subjected to shock or vibrations that can cause unwanted and damaging contact between the head and the surface of the disk. Also, parking the head 3 in the external parking zone provides additional space over the disk to store data.
This approach, sometimes referred to as xe2x80x9cramp loading,xe2x80x9d provides higher reliability because the head 3 is placed in a stable position and cannot hit the disk""s surface. Ramp loading requires an electronic circuit that can drive the arm 8 up the ramp to perform the parking operation and that can unpark the head to a smooth manner to control the landing of the head during a read/write operation to avoid crashing the head onto the disk surface.
In moving the head onto the ramp, sufficient voltage should be applied to the voice coil 6 to cause the arm 8 to be sufficiently moved to the parked position. At the same time, excessive voltages should not be applied to ensure that the head is not unnecessarily damaged by being overdriven into its parked position and to conserve power since the parking is often performed during power-down operations when power needs to be provided to other power-down functions. Determining the instantaneous velocity of the arm is difficult, however, because information generally is not instantaneously available from which instantaneous velocity can be determined.
A circuit to control the velocity of the head mechanism during park is disclosed in U.S. Pat. No. 5,566,369.
An embodiment of the invention provides a method to control a drive current to move a voice coil in a hard disk drive system. The method comprises setting a reference value, generating a signal representative of stored back electromotive force (BEMF) in the voice coil, generating a pulse width modulation (PWM) waveform, and applying the drive current based on the PWM waveform being modulated by either the reference value or a value of the signal representative of the stored BEMF.
In another embodiment, the method comprises modulating a periodic waveform to peak at either a reference level or at a signal associated with a stored BEMF in the voice coil. The method further comprises applying the drive current to the voice coil based on the modulated periodic waveform.
Yet another embodiment of the present invention provides an apparatus to control application of a drive current to a voice coil. The apparatus comprises a detection circuit coupled to the voice coil and structured to detect the actual BEMF in the voice coil and to generate a value representative of the BEMF. A sample and hold circuit is coupled to the detection circuit and structured to sample the BEMF generated by the detection circuit and store the sampled value. Comparing circuitry is coupled to the sample and hold circuit to compare a ramp waveform with a reference voltage and with the sampled value stored by the sample and hold circuit. A drive circuit is coupled to the voice coil and provides the drive current to the voice coil for a period of time corresponding to the increasing ramp portion of the waveform as it approaches either the sampled value or the reference voltage, whichever is less.